Lights having different wavelengths are propagated without interfering with each other. Thus, when signals are loaded in different wavelengths and transmitted through a single optical fiber, they are transferred through the optical fiber without interfering with each other. This transmission method is called wavelength division multiplexing (WDM). Information transmission capacity of optical fibers can be remarkably increased when the wavelength division multiplexing is used.
A dense wavelength division multiplexing technique is expected to be widely used in optical telecommunications for the purpose of commercial services. However, the dense wavelength division multiplexing technique increases the number of channels to thereby result in a formation of a network having a complicated structure. Furthermore, a light source having a stabilized wavelength recommended by ITU-T is required in order to design and operate a more flexible network.
Accordingly, International telecommunications Union (ITU) recommends the utilization of a light source having a channel frequency with equal spacing of 12.5 GHz, 25 GHz, 50 GHz or 100 GHz adjacent to 193.1 THz in order to use the dense wavelength division multiplexing. Recently, a variety of wavelength tunable lasers including a vertical cavity surface emitting lasers, an external cavity laser diode, a distributed feedback laser diode and a distributed Bragg reflector laser diode have been commercially developed and used in dense wavelength division multiplexing systems.
A wavelength tunable laser and a fiber laser should be designed such that their operating wavelengths correspond to a wavelength (frequency) recommended by ITU-T. The wavelength tunable laser and fiber laser are widely used as a light source in wavelength division multiplexing optical communication systems to effectively maintain the dense wavelength division multiplexing systems at a low cost.
An optical fiber laser developed as an application light source for testing elements at a high speed is attractive more than a semiconductor laser when applied to the dense wavelength division multiplexing systems because the optical fiber laser has a wide wavelength tunable range having excellent wavelength selectivity through a short wavelength band (S-band) of 1460 through 1530nm, a conventional band (C-band) and a long wave band (L-band) of 1530 through 1625 nm and a single mode operation.
TABLE 1TuningTuningLaserITU gridAuthorMechanismRangeLinewidthChannelsPapersM. Horowitzintracavitya few nm~kHz Electronics(C-band)Letters (1994)Y. ChengFBGa few nm0.95kHz Optics Letters(C-band)(1995)Y. SongFBG40 nm1kHz PTL (2001)(C-band)N. J. C. LibatiqueFBG, etalon50 GHz (8 Ch.)10KHz Opticsfilter(C-band)Express (2002)H. ChenTBF70 nm~kHz PTL (2003)(C + L-band)F. FarokhroozSagnac loop32 nm40kHz OFC (2004)filters(C-band)
Table 1 represents examples of fiber ring lasers using a saturable absorber, which have been studied and developed. All these fiber ring lasers are operated in a single mode. However, the fiber ring lasers restrict wavelength tunable ranges or do not operate to have a frequency separation recommended by ITU-T.